Thanks for spreading the word. Girls, stay healthy and go get that Vitamin D!
Posted on 06/21/2010 1:07:49 PM PDT by decimon
People who remain pale and never tan can blame their distant ancestors for choosing to live in the northern reaches of the globe and those who easily achieve a deep tan can thank their ancestors for living in the subtropical latitudes, according to Penn State anthropologists.
"The variation of ultraviolet radiation, especially in the middle and high latitudes is great," said Nina Jablonski, professor of anthropology and chair of Penn State's anthropology department. "Tanning has evolved multiple times around the world as a mechanism to partly protect humans from harmful effects of ultraviolet radiation."
Jablonski, working with George Chaplin, senior research associate in anthropology and an expert in geographic information systems, looked at the way the sun illuminates different parts of the Earth. They looked at levels and angles of incidence of both ultraviolet A and B radiation at various latitudes. Ultraviolet B radiation is much more variable than ultraviolet A as latitude increases due to atmospheric scattering of the light and absorption by oxygen.
Ultraviolet B radiation produces vitamin D in human skin. Ultraviolet radiation can, however, destroy folate. Folate is important for the rapid growth of cells, especially during pregnancy where its deficiency can cause neural tube defects.
"What we now recognize is that some of the medical problems seen in darkly pigmented people may be linked at some level to vitamin D deficiency," said Jablonski. "Things like certain types of cancer in darkly pigmented people and in people who use a lot of sunscreen or always stay inside could be partly related to vitamin D deficiency."
Scientists have understood for years that evolutionary selection of skin pigmentation was caused by the sun. As human ancestors gradually lost their pelts to allow evaporative cooling through sweating, their naked skin was directly exposed to sunlight. In the tropics, where human ancestors evolved and where both ultraviolet radiations are high throughout the year, natural selection created darkly pigmented individuals to protect against the sun.
"Past arguments about the selective value of dark pigmentation focused on the protective effects of melanin against sunburn, skin cancer, and overproduction of vitamin D. These factors can no longer be considered significant selective pressures," the Jablonski and Chaplin report in a recent issue of the Proceedings of the National Academy of Sciences.
Sunburn and most skin cancers do not alter an individual's ability to procreate, so they are not selection factors. The human body also has a mechanism to prevent overproduction of vitamin D.
Previously, the researchers concluded that dark skin pigmentation in the tropics protects people from folate destruction by ultraviolet B, but, because levels of ultraviolet B are high year round, the skin can still allow enough in to manufacture vitamin D.
As humans moved out of Africa, they moved into the subtropics and eventually inhabited areas up to the Arctic Circle. Ultraviolet radiation in these areas is neither consistent nor strong. North or south of 46 degrees latitude, which includes all of Canada, Russia, Scandinavia, Western Europe and Mongolia, there is insufficient ultraviolet B through most of the year to produce vitamin D. Populations in these areas evolved to have little skin pigmentation.
In the latitudes between 23 and 46 degrees, an area that encompasses North Africa, South America, the Mediterranean and most of China, ultraviolet B radiation is much more variable. Heavily pigmented skin in the winter would block the development of vitamin D, and lightly pigmented skin during the summer would allow destruction of folate.
"We actually demonstrate that in those middle latitudes where highly fluctuating levels of ultraviolet radiation occur throughout the year, tanning has evolved multiple times as a mechanism to partly protect humans from harmful effect of the sun," said Jablonski.
The tanning process evolved for humans who by and large were naked all the time. As the ultraviolet B radiation began to increase in the early spring, the skin would begin to gradually darken. As the sun became stronger, the tan became deeper. During the winter, as ultraviolet B waned, so did the tan, allowing Vitamin D production and protecting folate.
The researchers note that the ability to tan developed in a wide variety of peoples and while the outcome, tanablity, is the same, the underlying genetic mechanisms are not necessarily identical. They also note that depigmentated skin also developed at least three times through different genetic mechanisms.
Implications for today focus on the fact that depigmented people now live in tropical and subtropical areas where besides getting sunburned they run the risk of losing folate. Highly pigmented people live in higher latitudes where they may become vitamin D deficient, especially if they use sunscreens.
"It is a conspiracy of modernity," said Jablonski. "The rapidity at which we can move long distances and live far away from our ancestral homelands. The fact that we can live and work indoors. All this has happened within the last 500 years and especially within the last 200 years."
Wanderer ping.
Okay, here we go:
Father: dark brown hair, blue eyes, pale skin which burns easily and does not tan. Ancestors hail from England, Denmark, Germany and The Netherlands.
Mother: light brown hair, blue eyes, light skin with pale freckles. Tans only reluctantly. Ancestors from Scotland, Norway and Denmark.
Daughter #1: bright red hair, blue eyes, alabaster skin. Does not tan at all.
Daughter #2: brown hair, blue eyes, olive skin, tans fastastically.
No, there was no fooling around.
Oh, I’m the kind of guy who likes to roam around...
lol!
Somebody's just making up words.
When I worked construction jobs during high school and college the super came up to me once and said “If you get any darker, we’re going to start counting you as a minority”.
My yDNA (R1b) is traceable to Ireland and then earlier to Denmark. My mtDNA ('V') is traceable to the Skolt Sa'ami of northern Finland.
My dad's mother (Mrs Smith) is mtDNA (U5a) and is related to 9,000 year old Cheddar Man
Most Europeans are from haplogroup R1b (male) and 'H' (female) and after arriving from northern Pakistan spent most of the Ice Age in the Iberian region before expanding out over Europe after the Ice Age.
A real study is more like this one that actually measured Vitamin D production against skin color.
Wow! Where did you have that done and how much did it cost?
I hear you-One of my friends is 100% Irish and tans beautifully, while I am at least 25% Italian and only burn...Go figure.
It was $107.00 for each of the haplogroups.
The Genographic Project (Have Your DNA Checked, Find Your Roots)
By now all our DNA is quite watered down and I don’t think it is really that possible to pinpoint the ancestry of First World nations at least. In my example, we had girls of identical ancestry having drastically different tanning ability.
I haven’t had my DNA tested. I can trace my ancestry back ten to fifteen generations so far my ancesters and those of my husband are hanging out around the North Sea.
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Thanks decimon. "I'm drinking milk" ping. |
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Damn Irish!
Can I get a picture of daughter of daughter 2?
Thanks for spreading the word. Girls, stay healthy and go get that Vitamin D!
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